Although the invention is not limited to such applications, in the manufacture of phosphoric acid by the method described above, ground phosphate rock is continuously introduced with sulfuric acid in an agitated reaction volume containing the products of the main chemical reaction, viz., solid calcium sulphate and liquid phosphoric acid. These products are separated by filtration to recover the phosphoric acid while calcium sulphate, which may have different forms of hydration, is generally dumped as a waste material having no value.
The filter used in the liquid-solid separation process described above is an expensive piece of equipment which is critical to the process. In this regard, the separation must be as complete as possible in order to recover the maximum phosphoric acid. For this purpose, after extracting as much mother liquor as possible, the cake is washed several times (typically two or three times) in a countercurrent process with the make-up water which will end up in the reactor system as the process water needed to adjust the strength of the product phosphoric acid. The filter size is governed by the quantity and filterability of the solids and by the ability of the filter to drain the liquid as fast as possible to provide an increased filtration rate and sharper separation of filtrates of different qualities.
The present invention is related to horizontal circular filters of two types, tilting pan filters and table filters. The invention is particularly advantageous when applied to tilting pan filters because such filters can be easily and readily retrofitted to use the invention, and thus such tilting pan filters will be further described by way of additional background.
Referring to FIG. 1, the basic steps in a tilting pan filter cycle are shown. The filter, which is generally denoted 10, is circular as shown in FIG. 2, but, for purposes of clarity, is represented as a linear projection, in FIG. 1. Each pan 12 of the filter 10 receives a batch of slurry coming from the reactor tank. When most of the mother liquor has been extracted, as shown at the left of FIG. 1, the surface of the filter cake looks dry, and the cake is then washed with a solution of phosphoric acid at low concentration. The latter step is referred to as a first wash, and is indicated in FIG. 1, while the liquid extraction is referred to as dewatering. In a typical two wash filter, subsequent to dewatering after the first wash, the cake is again washed with water, in a step referred to as the second wash, and the filtrate is sent back to filter 10 for the first wash, as indicated by arrowed path 14. There may be more than two washes, depending on the water balance of the process. After the final dewatering, the pan tilts, as indicated for pan 12a in FIG. 1 (and is also shown in FIG. 2), to completely dump the solid filter cake, as indicated for pan 12b in FIG. 1. The filter cloth is then washed with water as indicated for pan 12c, and then the pan is again rotated, as indicated for pan 12d, so as to come back to the original upright position and thus be ready for a new cycle.
The filtrates for each stage of filtration, referred to the first, second and third filtrates in the example shown in FIG. 1, are collected separately. More specifically, referring to FIG. 2, wherein only a few pans 12 are shown for purposes of clarity, a manifold 16 is connected to rotate with pans 12 above a fixed circular vacuum box 18. The latter is divided by radial partitions to collect the filtrates of different qualities. Each filtrate has a different use in the process and it is important to sharply separate them.
After washing the filter cloths with water, the water must be drained from the pans 12 as thoroughly as possible before receiving the next batch of slurry, otherwise dilution occurs, i.e., the mother liquor is diluted with water. If dilution occurs, an expensive evaporation step is then required as the next step in the process. To obtain maximum filter efficiency in terms of capacity, mother liquor recovery and low dilution, it is important that the filtrates be drained as fast as possible.
To summarize and expand on the foregoing, the main required qualities of the filter, particularly for a phosphoric acid application, are the following: (1) to drain the filtrates rapidly from the filtration support to the collecting device so as to increase the production rate and improve the product acid quality. In this regard, because the first filtrate is contaminated by water not drained from the pans after the cloth wash and by solids going through the cloths before a pre-coat of solids is formed, the quantity of this first filtrate must be as small as possible to be discarded outside the product acid; (2) to provide about the same travelling time for the filtrates from any point of the pan surface to the collecting device since, otherwise, the separation of filtrates of different strengths recovered in the collecting device will not be sharp and will decrease both the filter capacity and the extraction efficiency; (3) to provide a cloth support with a ratio of void area to total surface area (void percentage) as high as possible so as to increase both the filtration rate and the extraction efficiency; (4) to provide minimum scaling of the pans such as occurs with cooling by precipitation of salts dissolved in the phosphoric acid; (5) to provide easy access to the bottom of the pans so as to enable mechanical removal of any scaling that is formed; and (6) to provide a small air "hold-up" for limiting the capacity of the vacuum pump removing this air.
Turning now to a consideration of prior art filter pans, originally the cloth support was a perforated rubber mat resting on the flat bottom of the pan. The filtrates traveled on this flat bottom to a corner of the pan where the filtrates were collected. This device had many disadvantages, including, in particular, the low drainage provided by the horizontal surface as well as scaling problems, and has been completely abandoned.
The device described above was replaced by a corrugated plate tack-welded on the bottom of the pan, thereby allowing the filtrate and the air to travel in narrow parallel channels at relatively high velocity. The device is efficient in providing the fast drainage of the filtrates, but suffers important disadvantages. For example, after operation over time, the corrugated plates have been found to be corroded, deformed and scaled. This device has also been abandoned due to these operational problems.
In the next device of interest, a sloped bottom of the pan is covered with a horizontal punched plate which is used as the cloth support with 35 to 45% void opening. Most of the circular filters in operation today in the phosphoric acid industry are of this type. The advantages over the former designs are that there is more room underneath the cloth support with less need for frequent cleaning and there is provided faster drainage of the filtrates than is provided by a horizontal flat bottom. The drawbacks include the following: difficulties in cleaning and removing the scale through the holes of the welded punched plates; the relatively low void ratio of the cloth support; vortices are formed in some holes perforating the cloths and even the pans; cracks occur in the punched plates due to alternative flexing thereof; and high manufacturing cost. Moreover, the apparent advantage of fast drainage is, in fact, offset by the build-up of scale which slows down the filtrates drainage.
In recent testing and industrial applications, a device has been used which includes a horizontal grooved polyethylene plate or grate supporting the cloth. The concept is similar to the corrugated plate described hereinabove. Industrial runs have shown that this device drains the filtrates faster than a sloped bottom pan and increases the productivity and efficiency of the filter. Compared to the stainless steel corrugated plate, this grate has the advantage of having less tendency to scaling due to the smoother material used, but has the disadvantage of a decreased void surface, typically 45%. After some industrial experience with this device, two further problems appeared: (1) the grates lift up due to buildup underneath thereof of salts and a poor fixation of the grates which is difficult to improve; and (2) a small hole in the cloth spreads rapidly due to the insufficient cloth support on too large channels. Other disadvantages include the relatively high cost of the grates due to the machining of the grooves required and the type of fixation needed, and leakage of liquid between the grate and the pan bottom results in scaling in this space and decreases the filter efficiency.
A more recent device used in industrial application is similar to the corrugated plate support device with the difference that the bottom of the pan itself is corrugated, in contrast to the thin tack welded corrugated plate described hereinabove. In order to limit the cooling effect due to the extended surface, the outside bottom of the pan must be thermally insulated with a special coating. Important disadvantages include the high manufacturing cost and the large surface exposed to scaling.
In all filter constructions described above, except for the abandoned rubber mat device, the filtrates coming from any point of the cloth are drained into a large sloped channel which is indicated at 20 in FIG. 2 and extends along the pan axis.